Our interest continues to be focused on the virus-host cell interactions that result in the induction of leukemia in mice. It is proposed to study the biologic and biochemical properties of the non-leukemogenic strains of Friend leukemia virus (FLV) and to compare them with those of the leukemogenic strains. We will also explore host factors which may influence the expression of leukemogenicity of the attenuated virus strains. It is hoped that precise cytological, immunological and biochemical studies of this model system may elucidate the mechanisms of malignant transformation and perhaps lead to an advance in our understanding of the nature and control of some of the neoplastic diseases in man.